** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** $Id: btree.c,v 1.582 2009/03/30 18:50:05 danielk1977 Exp $
**
** This file implements a external (disk-based) database using BTrees.
** See the header comment on "btreeInt.h" for additional information.
** Including a description of file format and an overview of operation.
*/
#include "btreeInt.h"

................................................................................
  BtShared *pBt = p->pBt;
  BtLock *pIter;

  assert( sqlite3BtreeHoldsMutex(p) );
  assert( eLock==READ_LOCK || eLock==WRITE_LOCK );
  assert( p->db!=0 );
  







  /* This is a no-op if the shared-cache is not enabled */
  if( !p->sharable ){
    return SQLITE_OK;
  }

  /* If some other connection is holding an exclusive lock, the
  ** requested lock may not be obtained.
................................................................................
  */
  if( 
    0==(p->db->flags&SQLITE_ReadUncommitted) || 
    eLock==WRITE_LOCK ||
    iTab==MASTER_ROOT
  ){
    for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){











      if( pIter->pBtree!=p && pIter->iTable==iTab && 
          (pIter->eLock!=eLock || eLock!=READ_LOCK) ){
        sqlite3ConnectionBlocked(p->db, pIter->pBtree->db);
        if( eLock==WRITE_LOCK ){
          assert( p==pBt->pWriter );
          pBt->isPending = 1;
        }
        return SQLITE_LOCKED_SHAREDCACHE;
      }

** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** $Id: btree.c,v 1.583 2009/04/01 09:41:54 danielk1977 Exp $
**
** This file implements a external (disk-based) database using BTrees.
** See the header comment on "btreeInt.h" for additional information.
** Including a description of file format and an overview of operation.
*/
#include "btreeInt.h"

................................................................................
  BtShared *pBt = p->pBt;
  BtLock *pIter;

  assert( sqlite3BtreeHoldsMutex(p) );
  assert( eLock==READ_LOCK || eLock==WRITE_LOCK );
  assert( p->db!=0 );
  
  /* If requesting a write-lock, then the Btree must have an open write
  ** transaction on this file. And, obviously, for this to be so there 
  ** must be an open write transaction on the file itself.
  */
  assert( eLock==READ_LOCK || (p==pBt->pWriter && p->inTrans==TRANS_WRITE) );
  assert( eLock==READ_LOCK || pBt->inTransaction==TRANS_WRITE );
  
  /* This is a no-op if the shared-cache is not enabled */
  if( !p->sharable ){
    return SQLITE_OK;
  }

  /* If some other connection is holding an exclusive lock, the
  ** requested lock may not be obtained.
................................................................................
  */
  if( 
    0==(p->db->flags&SQLITE_ReadUncommitted) || 
    eLock==WRITE_LOCK ||
    iTab==MASTER_ROOT
  ){
    for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){
      /* The condition (pIter->eLock!=eLock) in the following if(...) 
      ** statement is a simplification of:
      **
      **   (eLock==WRITE_LOCK || pIter->eLock==WRITE_LOCK)
      **
      ** since we know that if eLock==WRITE_LOCK, then no other connection
      ** may hold a WRITE_LOCK on any table in this file (since there can
      ** only be a single writer).
      */
      assert( pIter->eLock==READ_LOCK || pIter->eLock==WRITE_LOCK );
      assert( eLock==READ_LOCK || pIter->pBtree==p || pIter->eLock==READ_LOCK);
      if( pIter->pBtree!=p && pIter->iTable==iTab && pIter->eLock!=eLock ){

        sqlite3ConnectionBlocked(p->db, pIter->pBtree->db);
        if( eLock==WRITE_LOCK ){
          assert( p==pBt->pWriter );
          pBt->isPending = 1;
        }
        return SQLITE_LOCKED_SHAREDCACHE;
      }

# 2009 April 01
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
#
# $Id: shared6.test,v 1.1 2009/04/01 09:41:54 danielk1977 Exp $

set testdir [file dirname $argv0]
source $testdir/tester.tcl
ifcapable !shared_cache { finish_test ; return }

do_test shared7-1.1.1 {
  execsql {
    CREATE TABLE t1(a, b);
    CREATE TABLE t2(c, d);
    CREATE TABLE t3(e, f);
  }
} {}
do_test shared7-1.1.2 {
  set ::enable_shared_cache [sqlite3_enable_shared_cache 1]
  sqlite3_enable_shared_cache
} {1}

do_test shared7-1.1.3 {
  sqlite3 db1 test.db
  sqlite3 db2 test.db
} {}

# Exclusive shared-cache locks. Test the following:
#
#   1.2.1: If [db1] has an exclusive lock, [db2] cannot read.
#   1.2.2: If [db1] has an exclusive lock, [db1] can read.
#   1.2.3: If [db1] has a non-exclusive write-lock, [db2] can read.
# 
do_test shared7-1.2.1 {
  execsql { SELECT * FROM t1 } db2    ;# Cache a compiled statement
  execsql { BEGIN EXCLUSIVE } db1
  catchsql { SELECT * FROM t1 } db2   ;# Execute the cached compiled statement
} {1 {database table is locked}}
do_test shared7-1.2.2 {
  execsql { SELECT * FROM t1 } db1
} {}
do_test shared7-1.2.3 {
  execsql {
    COMMIT;
    BEGIN;
    INSERT INTO t2 VALUES(3, 4);
  } db1
  execsql { SELECT * FROM t1 } db2
} {}
do_test shared7-1.2.X {
  execsql { COMMIT } db1
} {}

# Regular shared-cache locks. Verify the following:
#
#   1.3.1: If [db1] has a write-lock on t1, [db1] can read from t1.
#   1.3.2: If [db1] has a write-lock on t1, [db2] can read from t2.
#   1.3.3: If [db1] has a write-lock on t1, [db2] cannot read from t1.
#   1.3.4: If [db1] has a write-lock on t1, [db2] cannot write to t1.
#   1.3.5: If [db1] has a read-lock on t1, [db2] can read from t1.
#   1.3.6: If [db1] has a read-lock on t1, [db2] cannot write to t1.
#
do_test shared7-1.3.1 {
  execsql {
    BEGIN;
    INSERT INTO t1 VALUES(1, 2);
  } db1
  execsql { SELECT * FROM t1 } db1
} {1 2}
do_test shared7-1.3.2 {
  execsql { SELECT * FROM t2 } db2
} {3 4}
do_test shared7-1.3.3 {
  catchsql { SELECT * FROM t1 } db2
} {1 {database table is locked: t1}}
do_test shared7-1.3.4 {
  catchsql { INSERT INTO t2 VALUES(1, 2) } db2
} {1 {database table is locked}}
do_test shared7-1.3.5 {
  execsql {
    COMMIT;
    BEGIN;
    SELECT * FROM t1;
  } db1
  execsql { SELECT * FROM t1 } db2
} {1 2}
do_test shared7-1.3.5 {
  catchsql { INSERT INTO t1 VALUES(5, 6) } db2
} {1 {database table is locked: t1}}
do_test shared7-1.3.X {
  execsql { COMMIT } db1
} {}

# Read-uncommitted mode.
#
# For these tests, connection [db2] is in read-uncommitted mode.
#
#   1.4.1: If [db1] has a write-lock on t1, [db2] can still read from t1.
#   1.4.2: If [db1] has a write-lock on the db schema (sqlite_master table), 
#          [db2] cannot read from the schema.
#   1.4.3: If [db1] has a read-lock on t1, [db2] cannot write to t1.
#
do_test shared7-1.4.1 {
  execsql { PRAGMA read_uncommitted = 1 } db2
  execsql {
    BEGIN;
    INSERT INTO t1 VALUES(5, 6);
  } db1
  execsql { SELECT * FROM t1 } db2
} {1 2 5 6}
do_test shared7-1.4.2 {
  execsql { CREATE TABLE t4(a, b) } db1
  catchsql { SELECT * FROM t1 } db2
} {1 {database table is locked}}
do_test shared7-1.4.3 {
  execsql {
    COMMIT;
    BEGIN;
    SELECT * FROM t1;
  } db1
  catchsql { INSERT INTO t1 VALUES(7, 8) } db2
} {1 {database table is locked: t1}}

do_test shared7-1.X {
  db1 close
  db2 close
} {}

sqlite3_enable_shared_cache $::enable_shared_cache
finish_test